Concrete industrial floor slabs usually rest via a foundation layer on a natural ground. Unevenly distributed loads on top of the floor slab are transmitted via the floor slab and the foundation layer in a more evenly distributed form through to the natural ground, which eventually bears the load.
Natural grounds of an inferior quality, e.g. characterized by a Westergaard K-value of less than 10 MPa/m, are first dug up and/or tamped down and leveled before the foundation is laid over it.
Due to the fact that a lot of natural grounds of good quality (characterized by a high Westergaard K-value) have already been taken for existing constructions, the number of natural grounds with inferior or even unacceptable quality (i.e. with a low Westergaard K-value) which are being considered for constructions is increasing. The bearing capacity of some grounds is so bad that digging up and/or excavating and/or tamping down would constitute an enormous amount of work and cost.
In such a case it is known to rest the floor slab on driven or bored piles. Placing a floor slab on driven or bored piles under load, however, creates very high negative peak moments in the areas above these piles and relatively much lower (about one fifth of the height of the peak moments) positive moments in the zones between the piles. Reinforcing floor slabs on driven or bored piles with uniformly distributed steel fibres would not be economical since the zones between the piles would have a quantity of steel fibres which is unnecessarily too high and which would cause trouble during the pumping and pouring of the concrete and would render the solution not economical.
This problem has been solved in FR 2 718 765 of applicant, by having the floor slab rest on a number of gravel columns. As has been explained therein, these gravel columns are not as rigid as common piles and compress relatively easily under a downward load (the compression modulus of gravel columns e.g. ranges from 0.2 to 0.4 MN/cm) so that the gravel columns function like a spring in a mathematical model, which means that the floor slab is no longer subjected to high bending deformations in the zones above the columns.
In the international application PCT/EP98/00719 of applicant a solution has been disclosed to the above-mentioned problem. The present invention involves an improvement of the invention disclosed in this international application.